A Crisis at the Edge of Physics
nytimes.com
nytimes.com
For example, people working on string theory and people working on say, quantum loop gravity are somewhat at odds with one another. String theory may not see the QLG people as a direct threat but the QLG people certainly sees string theory as competition. Now you may wonder, "In competition for what?" and the answer to that is a bit more obvious: Funding (money for research and their very own paychecks) and attention from their peers/media/press/etc. These people's very livelihoods are at stake.
If QLG was proven wrong tomorrow, it's not like the people who were doing that for the last 15 years can just jump into another field. There's plenty of incentive for "hostile science" as I like to call it. There have been some well-known physicists who have written entire books bashing their competition for this purpose (See: Peter Woit's "Not Even Wrong", which incidentally, is chuck full of so many inaccuracies someone else wrote a book disputing his book...)
So when I see articles like these, I like to check out who the author is. Do they have a reason to write this piece? In this case, it's Adam Frank and Marcelo Gleiser. Both are coincidentally astrophysicists at very respected universities. I wonder what they're working on?
It appears Marcelo Gleiser, one of the authors, just published an anti-String theory book which claims that "We don't need a Theory of Everything".
In an ideal world the argument should stand alone, but I find people's motives to be worth considering. Not for the usual you'll find in the media like looking for the corrupt or using those motives to dismiss their arguments. I find how people come to their beliefs to be really interesting. I can learn from the path they took even if I find the result to be broken. The danger is admiring the path and supporting the broken argument.
As to the politics. Two people interacting means that you will have politics. Humans have conflicts from the battlefield to the soapbox to the pulpit to the lecture hall.
If the arguments are actually good, it doesn't matter what motivated a person to make them. Casting aspersions on motive is just a cheap way people avoid dealing with good arguments they don't like, perhaps because of subtle biases of their own.
Also, it's "chock-full" not "chuck full" (I had to look it up and I had another wrong answer for the correct spelling, so I thought it worth mentioning.)
If we accept this premise and consider this a sensible card to play, in the spectrum of physics resource allocation (say <5%), then these theorists need to start somewhere. Physics beyond the standard model is fertile ground, as is dark matter and friends. (not-so?) Friendly competition for these marginal portions of physics funding might be a good thing.
Aside: It borders on the surreal to me that these ideas need to be taken to the public to secure funding. The amount of abstraction between popular accounts of modern physics and the math itself is profound. The language used to talk about these theories begins to sound like fairy tales.
Sounds like string theory people really knows how to play this game.
We need evidence backing up what we take to be true.
But we can and must be able to work on ideas without evidence for their truth. We must be given time to flash them out, to try and find ways to get evidence for them.
Wanting to pursue a possible explanation is not the same as believing that it's true.
Many of the people who work on string theory obviously believe it to be true in one way or another; it's hard to stay motivated otherwise. But the rest of the world need not care whether it is true or not. For the time being, it's just one untested hypothesis among many.
It can take years (or even decades) for a theory to grow to the point where it can produce testable hypotheses, and more decades (sometimes even centuries) to test those hypotheses. This is especially true for theories that deal with extreme scales of space and/or time, such as cosmology, geology, paleontology, and (at the other extreme) anything that deals with subatomic particles.
Gravitational waves [1], for example, have been predicted for almost a century now, but we still don't seem to know how to build a machine to detect them.
The last time this topic came up, I wrote a response titled "Why so impatient?" I'll leave the link below [2] in case anyone is interested.
Long term thinking is really hard to do these days.
I'm 36 and my generation seems to take for granted that everything evolves at the same pace as most 20th century technologies have done so far.
This seems to be compounded by the prevalent market driven logic that prioritizes short term profit above all.
Putting these things in their correct historic timescale is very insight-full.
thank you =)
It's been a decade since that, and the experimental evidence for string theory remains nonexistent. As Fred Hoyle once wrote, "Science is prediction, not explanation". A theory with no experimental support cannot lead to usable technology, either.
Fundamental physics is currently stuck. There's a lot of denial about this. There's a whole generation of string theory faculty in senior positions. That's the problem.
[1] http://www.theguardian.com/science/2006/oct/08/research.high...
The other problem for physics is that so much of the talent works on Wall St doing things that are - for all practical purposes - useless, if not counterproductive.
So physics is stuck for two reasons, and won't start moving again until the culture changes enough to fix both problems.
Given the declining (political) value of long-term theoretical research in academia, and the relative impossibility of doing an original PhD exploring ideas outside the mainstream, I'm not expecting change any time soon.
This is beyond tragedy, because in terms of medium-term human survival, nothing is more important than new science.
One can hardly blame theoretical physicists for exploring above and beyond what the experimentalists are doing. It's not like there's a large body of more practical, down-to-earth problems being ignored in favor of these sexier, Nobel-bait questions.
I believe the point is that we are in trouble if something exciting doesn't happen at LHC; in this case, getting the planet to fund an O($1T) accelerator to probe higher energy levels seems far-fetched. Theorists know this. This is a closely related point to the comment about the politics of theory.
I don't know that this automatically spells crisis. There are plenty of problems out there that require the mathematical talent of folks that can perform at this level. Eventually, when more data is available, the pendulum will swing back towards interesting theory.
After all, it's not the case that if there isn't some rush to a GUT before 2020, the human race loses.
Physicists need to get their heads out of their political/philosophical asses and start earning their bread - which is nothing less than to justify our technological existence by discovering the knowledge that will help our species live longer. If not for this, I think we'd all be better off living on a planet of 200M people living in rural villages, leading simple lives that are as happy and fair and erudite as fate and culture allows.
Quantum decoherence is different: it merely follows the equations, and do not posit any additional entity on top of them (such as a collapse). The consequence of removing that entity is multiple universes, but so what?
The problem with science is that it tends to favour the first theory that fits the fact. Instead, it should favour the simplest theory that fits the fact. http://lesswrong.com/lw/qa/the_dilemma_science_or_bayes/
Many World Interpretation is not like the epicycles. It is like the ellipses. It's the Copenhagen interpretation that is like the epicycles, by postulating a collapse that the equations don't mention at all. The MWI is just taking the equations at face values. Postulating a collapse on top of that makes a more complex theory.
If many world came first, the collapse postulate would just be laughed at. "You're postulating a collapse in just the parts of our universe we can't observe? That reeks of "if I can't see it, it doesn't exists. If you're going to push that theory, you'd better produce empirical evidence."
By the way, I don't get the obsession with quantum mechanics interpretations. Copenhagen and many worlds give exactly the same predictions, so if one is correct then so is the other. Who cares how you choose to interpret it. By analogy to probability theory, Copenhagen is like conditioning on an observation, many worlds is like looking at the whole probability distribution. These are just two ways of looking at the same thing. It's as silly as arguing whether the Hamiltonian or Lagrangian formulation of classical mechanics is correct. Both are.
Not quite. While they predict the same observations, they certainly don't predict the same universe. Under the Copenhagen interpretation, there is only one cat, who is either dead or alive. The possibility you don't see doesn't even exist, the collapse has seen to that.
We can make an analogy with the expansion of space being faster than light. Let's say you send a life ship far away into deep space to do some colonisation. Let that ship travel beyond our observable bubble (it's a very high tech ship).
So, once your ship is so out of reach that it can't even send any signal back (not even in theory), does it still exist? If you take the current laws of physics at face value, it's still out there. The colonists are on their own, but they should be fine. On the other hand, if there is some kind of "collapse" where anything that goes beyond our observable bubble just disappears, then you have sent the colonists to their death. Oops.
For the record, I must say I am very uncomfortable about having the fundamental constants of the universe change as we go beyond our observable bubble. That sounds like an additional assumption, and I don't like it at all. I'd sooner believe in a Tegmark level IV multiverse.
By the way, many worlds by itself is not actually a complete theory. It simply says that the wavefunction of the entire universe evolves according to the rules of quantum mechanics. To actually get predictions out of it you have to say something extra about observers within that universe. We certainly don't experience multiple simultaneous versions of ourselves, but we do experience multiple simultaneous versions of the things around us (e.g. double slit experiment). You need additional rules about what an observer in the universe will see and with which probabilities.
Crap. Okay, I'll keep that in mind.
> Then the question of whether that spaceship still exists is not a sensible question in physics, because there is no experiment that can confirm or deny it.
We could say it's not a sensible question in science (no experimental difference), but I think this is still a very important ethical question: I would still care about whether the colonists live or die.
Long term, this could be a very practical question: how should we expand? Must we stay within reach, or can we safely go as far away as possible? At this point, I don't really care if it's a metaphysical question. From the look of it, there's a definite answer, and one which will influence expansion policy a great deal.
Though to be fair, the point is kinda moot until we have a theory of everything.
The latter is a rather more concrete affair, basically saying that in regions of the universe too far away to be observable, the parameters of low energy physics (kinds of particles, strengths of interactions between them) can be different from what we see in our region; rather than being fundamental properties of physics, they were picked randomly (by physical processes) at the big bang. If they are not fundamental properties of the theory, there is no need to come up with a theory which explains their values (the fine-tuning problem mentioned in the essay); we live in a region where the values are such that our existence is possible, because that is the only possibility. In regions where the values do not allow the existence of observers capable of asking "why are the values such that I can exist?", there are no observers asking "why are the values such that I can exist?".
Some call this idea (the anthropic principle) neat, others call it a cop-out and point out that since the postulated regions with different physical parameters can not be observed, the whole construction falls outside the scope of empiricism, and so can not be science (as traditionally understood). Hence essays such as this...
I did imply I did not know what multiverse was being considered. As far as I can tell, the article did not say which multiverse it was talking about. There are many reasons to believe in an… unbelievably large universe:
1) Beyond the observable universe. Beyond a certain limit, the universe expands so fast that even light from there can't reach us, ever. Beyond that limit, it might as well be another universe. But whatever lies beyond that limit aren't additional entities. They're just the consequence of known laws of physics. Positing that they somehow don't exist would form an additional assumption in the theory, and therefore not good from an Occam's razor perspective.
2) Inflation. Would apparently create a number of "bubbles" or something, that are sufficiently far a part not to observe each other. I can't judge this one.
3) Macroscopic decoherence. The particle is in in a superposition of being destroyed/intact, the cat is in a superposition of being dead/alive, the scientist that observe the cat is in a superposition of mourning/petting the cat… Well, the Many World Interpretation of quantum mechanics. Well, that's what we call the Many World Interpretation of quantum mechanics. Again, no additional entity here: the other universes are just a natural consequence of long known equations. If anything, we remove an entity, compared to the Copenhagen interpretation: that pesky collapse.
4) Tegmark's level IV multiverse, where every possible mathematical construct "exists" in some sense, and our universe is just one of them (which also happen to support sentient life). Right now, I don't know what to think of it. Though it would be incredibly convenient, from an anthropic principle stand point.
> since the postulated regions with different physical parameters can not be observed,
Are we postulating the regions, or are we postulating a large universe with changing parameters? This is not the same thing. The former is obviously incredibly complex, and therefore a priori impossibly improbable. The latter doesn't involve that many entities, and may even be simpler than current mainstream theories. Or it may not. I'm not a physicist.
You do bring up a valid point about occam's razor - i.e. It is a valid effort to come up with a strictly simpler theory that can explain already known observations than what is currently accepted. "Simpler" in the sense of fewer assumptions being needed. The difficulty is that we're often blind to the assumptions we're making in our theories and it can take a while for those to surface.
Any new theory in physics must have at least the same precision as the known theories. The problem in current physics-at-the-edge (raised by the article) is that these newer theories are coming up with so many ways in which universes can be formed that even though we're moving in the "simpler" direction by relinquishing fundamental some assumptions, we're losing "precision" in this sense. This is now happening to the extent that the very notion that the job of a scientific theory is to only explain the observable universe is being called into question and theories that consider our universe to be just one of many universes with different parameters are also included in the play. At that point, the working assumption that all these universes that our mathematics invents actually exist though we cannot observe them, is a simpler view of the cosmos ... until we know better. However, the practical utility of such an expansive theory ends up being limited relative to the theory that it is trying to replace, since our ability to predict things in our universe is not being improved on.
edit: What I'm trying to say is, I guess, that "precision" and "simplicity" are opposing forces that keep a check on overfitting.
Yes, OP's article is talking about bubble-universes, the lesswrong article is talking about the Many Worlds Interpretation. I think parent comment is talking about MWI as well.
For example, explaining observations using multiple-worlds QM is very simple in terms of the rules used, but has a very complex "location" within that theory (ie. pin-pointing which Universe those observations are being made in).
There's a nice overview of this idea in http://www.mdpi.com/1999-4893/3/4/329
Without it, how could you know what's true?
If a line of physical theory never leads to something testable, is it a physical theory?
Elegance and beauty are major guiding lights for theory, but they can't be the end goal if our desire is to learn the nature of Nature.
Arkani-Hamed and others chasing the physical utility of the Grassmanian are a modern example of beautiful mathematics straining toward physically-testable theory. I expect you'll see more money flowing that way in the next few years.